Some have posted vests that are rated at 50n (n=newtons). Impact vests are around 50n, so if you want more floatation, avoid 50n vests.

Liquid Force's Profile kite vest has the most amount of buoyancy for a waist harness compatible vest.
LF's Vortex Kite vest is CG approved and will offer the most amount of floatation and is designed to fit over a waist harness.
I also agree that getting a seamstress/tailor to add some pockets would be your best option. Or you could modify a waist pack to mount to your harness or kiteboard.